Magnetic cores used in motors, transformers, and the like are required to have high magnetic flux density and low iron loss. Conventionally, electrical steel sheets have been stacked in such magnetic cores, yet in recent years, dust cores have attracted attention as magnetic core material for motors.
The most notable characteristic of a dust core is that a 3D magnetic circuit can be formed. Since electrical steel sheets are stacked to form a magnetic core, the degree of freedom for the shape is limited. A dust core, on the other hand, is formed by pressing soft magnetic particles coated with insulating coating. Therefore, all that is needed is a die in order to obtain a greater degree of freedom for the shape than with electrical steel sheets.
Press forming is also a shorter process than stacking steel sheets and is less expensive. Combined with the low cost of the base powder, dust cores achieve excellent cost performance. Furthermore, since the surfaces of the stacked steel sheets are insulated, the magnetic properties in the steel sheet surface direction and the direction perpendicular to the surface differ, causing electrical steel sheets to have the defect of poor magnetic properties in the direction perpendicular to the surface. By contrast, in a dust core, each particle is coated with insulating coating, yielding uniform magnetic properties in every direction. A dust core is therefore appropriate for use in a 3D magnetic circuit.
Dust cores are thus indispensable material for designing 3D magnetic circuits, and due to their excellent cost performance, they have also been used in recent years from the perspectives of reducing the size of motors, reducing rare earth elements, reducing costs, and the like. Research and development of motors with 3D magnetic circuits has thus flourished.
When manufacturing high-performance magnetic components using powder metallurgy techniques, the components are required to be high density and to have excellent iron loss properties after formation. By increasing density, the magnetic flux density and the magnetic permeability of the iron core increase, allowing for generation of high torque with low current. Furthermore, reducing iron loss improves motor efficiency.
Against the above-described background, a variety of high compressibility iron powders have been developed. For example, JP 2007-92162 A (PTL 1) and WO 2008-093430 (PTL 2) disclose techniques related to a high compressibility iron powder that includes by mass %, as impurities, C: 0.005% or less, Si: more than 0.01% and 0.03% or less, Mn: 0.03% or more to 0.07% or less, P: 0.01% or less, S: 0.01% or less, O: 0.10% or less, and N: 0.001% or less, wherein a particle of the iron powder has an average crystal grain number of 4 or less and a micro Vickers hardness Hv of 80 or less on average.
JP H06-2007 A (PTL 3) discloses pure iron powder for powder metallurgy with excellent compressibility and magnetic properties. The impurity content of the iron powder is C≤0.005%, Si≤0.010%, Mn≤0.050%, P≤0.010%, S≤0.010%, O≤0.10%, and N≤0.0020%, the balance being substantially Fe and incidental impurities. The particle size distribution is, on the basis of weight percent by sieve classification using sieves prescribed in JIS Z 8801, constituted by 5% or less of particles of −60/+83 mesh, 4% or more to 10% or less of particles of −83/+100 mesh, 10% or more to 25% or less of particles of −100/+140 mesh, and 10% or more to 30% or less of particles passing through a sieve of 330 mesh. Crystal grains included in particles of −60/+200 mesh are coarse crystal grains with an average grain size number of 6.0 or less as measured by a ferrite grain size measuring method prescribed in JIS G 0052. When 0.75% of zinc stearate is blended as a lubricant for powder metallurgy and the result is compacted with a die at a compacting pressure of 5 t/cm2, a green density of 7.05 g/cm3 or more is obtained.
Furthermore, JP 4078512 B2 (PTL 4) discloses a high compressibility iron powder 1 such that the particle size distribution of iron powder is, on the basis of mass % by sieve classification using sieves prescribed in JIS Z 8801, constituted by more than 0% to 45% or less of particles that pass through a sieve having a nominal dimension of 1 mm and do not pass through a sieve having a nominal dimension of 250 μm, 30% or more to 65% or less of particles that pass through a sieve having a nominal dimension of 250 μm and do not pass through a sieve having a nominal dimension of 180 μm, 4% or more to 20% or less of particles that pass through a sieve having a nominal dimension of 180 μm and do not pass through a sieve having a nominal dimension of 150 μm, and 0% or more to 10% or less of particles that pass through a sieve having a nominal dimension of 150 μm; the micro Vickers hardness of iron powder particles that do not pass through the sieve having a nominal dimension of 150 μm is 110 or less; and the impurity content of the iron powder is, by mass %, C≤0.005%, Si≤0.01%, Mn≤0.05%, P≤0.01%, S≤0.01%, O≤0.10%, and N≤0.003%. PTL 4 also discloses a high compressibility iron powder 2 such that the particle size distribution of iron powder is, on the basis of mass % by sieve classification using sieves prescribed in JIS Z 8801, constituted by more than 0% to 2% or less of particles that pass through a sieve having a nominal dimension of 1 mm and do not pass through a sieve having a nominal dimension of 180 μm, 30% or more to 70% or less of particles that pass through a sieve having a nominal dimension of 180 μn and do not pass through a sieve having a nominal dimension of 150 μm, and 20% or more to 60% or less of particles that pass through a sieve having a nominal dimension of 150 μm; the micro Vickers hardness of iron powder particles that do not pass through the sieve having a nominal dimension of 150 μm is 110 or less; and the impurity content of the iron powder is, by mass %, C≤0.005%, Si≤0.01%, Mn≤0.05%, P≤0.01%, S≤0.01%, O≤0.10%, and N≤0.003%.